1. Field of the Invention
The present invention generally relates to the field of optical fibers, in particular the present invention is directed to using ultrasound to aid in the curing of UV-curable coatings of optical fibers.
2. Discussion of Related Art
Optical fibers are very small diameter glass strands which are capable of transmitting an optical signal over great distances, at high speeds, and with relatively low signal loss as compared to standard wire or cable networks. The use of optical fibers in today's technology has developed into many widespread areas, such as: medicine, aviation, communications, etc. Because of this development, there is a growing need to produce optical fibers of better quality at faster rates and lower costs.
Many of the areas of use for optical fibers, such as communications, require the optical fibers be protected from various destructive elements, such as adverse weather, moisture, impact damage, etc. This protection for the individual fibers comes from the fiber coatings. Today, most optical fibers have two coatings, which are often referred to as the primary and secondary coatings. The primary coating is applied onto the surface of the optical fiber, with the secondary coating being applied on top of the primary coating. The main function of the primary coating is to provide a soft “cushion” for the glass fiber, protecting it from shock damage. The main purpose of the secondary coating is to provide a semi-rigid protective shell to protect both the primary coating and the glass fiber from adverse environmental elements, as well as physical damage.
One of the most common methods of making optical fibers today is by a process often referred to as the “draw” process. In this process, a large glass preform is made. The preform is the actual material that the glass fiber (optical fiber) is made from. Once the preform is made, the next step is to “draw” the preform into a glass fiber (optical fiber) with the desired diameter. One of the most common means to accomplish this is through the use of a “draw tower”. The “draw tower” is a production apparatus which has all of the major stages required to manufacture an optical fiber from a glass preform to the finished fiber. In this process the glass preform is typically suspended above the apparatus with the bottom most end of the preform entering into a furnace. The furnace uniformly melts the preform such that the preform exits the furnace as a very thin diameter optical glass fiber. The rate the preform is moved into the furnace can be regulated to allow the maintenance of a constant diameter in the optical fiber. Once the glass fiber leaves the furnace it is generally cooled.
After the glass fiber is cooled to a preset temperature then the fiber is typically coated with the primary coating. This is generally done in a coating die. The primary coating is applied in such a way as to completely cover the fiber. The primary coating is then cured or hardened. Once the primary coating has been cured or hardened, the secondary coating is then applied to completely cover the primary coating. The secondary coating is then cured so as to harden it and secure it to the primary coating. Once this process is complete then the fiber is generally considered an optical fiber, as commonly known and understood. Finally the optical fiber is wound past a capstan and onto a reel or spool.
The coatings of the optical fiber are mainly used to provide chemical, mechanical and environmental protection to the glass fiber core and cladding. To accomplish this purpose the two layers are usually made from different materials. Generally the primary coating is relatively soft (having a relatively low Modulus of Elasticity of 1–2 MPa) when compared to the secondary coating and is used as a cushion or shock protection for the glass fiber. The secondary coating is relatively hard (having a relatively high Modulus of Elasticity of 30–60 MPa) and provides a semi-rigid protective shell for the fiber and primary coating. The most common types of coatings used are ultraviolet (UV) curable coatings. These are coatings which have a photoinitiator component used in the coating composition which allow the curing of the coatings to be initiated by exposure to UV radiation.
Photoinitiators function by absorbing energy which is radiated by a UV, or sometimes a visible, light source. This energy absorption then initiates polymerization of the liquid coating placed on the fiber, and results in the hardening of the coating. The fast cure of coatings greatly reduces the production time of optical fibers, making production more profitable.
However, this method of curing optical fiber coatings is not without its problems. Among other things, the curing process can generate a large amount of heat in the coatings of the fiber. This heat generally comes from hot UV/Visible lamps by convection or by infrared irradiation accompanying the UV- or visible light of a lamp during cure, and from the exothermic polymerization (i.e. cure) itself. This heat also contributes in curing the coatings, but can cause serious problems. For instance, if the temperature of coating during cure is too high it may result in decomposition of microradicals in the coating material and result in a low degree of cure. Overall, it is generally known that excessive heat during the coating cure process is detrimental for efficient or effective cure by free radical polymerization.
As stated earlier, the primary coating should be relatively soft, to protect the glass fiber from microbending. Microbending is the formation of microscopic bends in the glass fiber, which will reduce the effectiveness of the fiber by reducing the magnitude of transmitted light power, i.e. attenuation. In the cure process the primary coating is applied and then cured, and then the secondary coating is applied and then cured. As the secondary coating is being cured, the primary coating is, and often additionally, cured up until 100% conversion of the coating monomer(s) into polymers.
Because of the problems associated with the generation of excessive heat during the cure process, many prior art methods have been developed to expedite the cure process without the generation of excessive heat. An example would be to reduce the level of UV radiation, however, with the reduction of UV radiation the line speed must be slowed to ensure that the coating receives the proper amount of UV exposure to effect a proper cure. This reduction in draw speed can severely affect the manufacturing efficiency of a fiber optic facility. It is desired to have a system of curing optical fiber coatings at as high a speed as possible.